Spinach (Spinacia oleracea) is a flowering plant from the Amaranthaceae family that is grown as a vegetable. The consumable parts of spinach are the leaves from the vegetative stage. Spinach is sold loose, bunched, in pre-packed bags, canned, or frozen. There are three basic types of spinach, namely the savoy, semi-savoy and smooth types. Savoy has dark green, crinkly and curly leaves. Flat or smooth leaf spinach has broad, smooth leaves. Semi-savoy is a variety with slightly crinkled leaves. The main market for spinach is baby-leaf. Baby spinach leaves are usually of the flat-leaf variety and usually the harvested leaves are not longer than about eight centimetres. These tender, sweet leaves are sold loose rather than in bunches. They are often used in salads, but can also be lightly cooked.
Downy mildew—in spinach caused by the oomycete fungus Peronospora farinosa f. sp. spinaciae (formerly known as Peronospora effusa)—is a major threat for spinach growers, because it affects the harvested plant parts, namely the leaves. Infection is economically devastating, as it makes the leaves unsuitable for sale and consumption, as it manifests itself phenotypically as yellow lesions on the older leaves, and on the abaxial leaf surface a greyish fungal growth can be observed. The infection can spread very rapidly, and it can occur both in glasshouse cultivation and in soil cultivation. The optimal temperature for formation and germination of Peronospora farinosa f. sp. spinaciae spores is 9 to 12° C., and it is facilitated by a high relative humidity. When spores are deposited on a humid leaf surface they can readily germinate and infect the leaf. Fungal growth is optimal between 8 and 20° C. and a relative humidity of ≥80%, and within 6 and 13 days after infection mycelium growth can be observed. Oospores of Peronospora farinosa can survive in the soil for up to 3 years, or as mycelium in seeds or living plants.
In recent years various resistance genes (so-called R-genes) have been identified that provide spinach plants with a resistance against downy mildew. However, it has been observed that previously resistant spinach cultivars can again become susceptible to the fungus. Investigations revealed that the cultivars themselves had not changed, and that the loss of downy mildew resistance must therefore be due to Peronospora farinosa overcoming the resistance in these spinach cultivars. The downy mildew races (also called physios, pathogenic races, or isolates) that were able to infect resistant spinach cultivars were collected in a differential reference set, which can be used to test spinach cultivars for resistance. The differential set comprises a series of spinach cultivars (hybrids) that have different resistance profiles to the currently identified pathogenic races.
Even though R-genes are extensively used in spinach breeding, until now not much is known of these R-genes. The R-genes present in the current commercial spinach varieties have never been characterized at the molecular level, i.e. their genomic sequence until now was unknown. Up until now there are no closely linked molecular markers known in the art that separate these R-genes, nor are the molecular characteristics of the genes themselves known in the art. Therefore, the search for new R-genes and R-gene identification is currently based on phenotypic assays in which many accessions are screened for possible variation in their resistance pattern. Subsequently it has to be determined through crossing and selection whether a newly observed resistance is in fact caused by an R-gene.
To date 16 pathogenic races of spinach downy mildew (Pfs) have been officially identified and characterized. Races 4 through 10 have been identified between 1990 and 2009 (Irish et al., 2008, Phytopathol. 98: 894-900), which illustrates the versatility and adaptability of the fungus to overcome resistances in spinach. In different geographical regions different combinations of pathogenic races occur, and the spinach industry therefore has a strong demand for spinach cultivars that are resistant to as many relevant downy mildew races as possible, preferably to all races that may occur in their region, and even to the newest threats that cannot be countered with the resistances that are present in the commercially available spinach cultivars.
In March and August 2011, the “International Working Group on Peronospora farinosa” (IWGP) designated two isolates as the type isolates for new races Pfs12 and Pfs13, respectively. As illustrated by Table 1, these newly identified Peronospora races can break the resistance of many spinach varieties that are currently used commercially worldwide, and they thus pose a serious threat to the productivity of the spinach industry. Since 2012, three new Peronospora isolates have been officially named as pathogenic races: UA4410 has been termed Pfs14 in 2012, UA4712 has been named Pfs15 in 2014, and UA1519B has become Pfs16 in 2016.
These 16 officially recognised Pfs races are all publicly available from the Department of Plant Pathology, University of Arkansas, Fayetteville, Ark. 72701, USA, and also from NAK Tuinbouw, Sotaweg 22, 2371 GD Roelofarendsveen, the Netherlands.
Spinach variety Viroflay is an example of a spinach line that is susceptible to all known Peronospora farinosa f. sp. spinaciae physios, while cultivars such as Lion and Lazio show resistance to multiple pathogenic races. However, it is crucial to stay at the forefront of developments in this field, as Peronospora continuously develops the ability to break the resistances that are present in commercial spinach varieties. For this reason new resistance genes are very valuable assets, and they form an important research focus in spinach breeding. The goal of spinach breeders is to rapidly develop spinach varieties with a resistance to as many Peronospora farinosa races as possible, including the latest identified races, before these races become wide-spread and can threaten the industry.
In the prior art no single resistance gene (R-gene) is known that confers resistance to all the known physios. In the absence of a suitable resistance to counter this pathogenic threat, especially the new isolates may spread during the next growing seasons and cause great damage to the worldwide spinach industry in the immediate future.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.